There exists an increasing need for systems that are capable of supplying oxygen at very high pressures in which the oxygen exists as a supercritical fluid, namely, a fluid that is neither a vapor, solid or liquid, but is rather a dense fluid having a temperature and pressure above the supercritical point. For oxygen, this temperature and pressure would be above 154.78 K and 50.83 bar (a).
One reason for this increasing need is in the growth of gasification applications. Gasification is an environmentally friendly technology which can utilize coal or other relatively low value feedstocks and convert them into high-value products, or alternatively produce a clean source of electrical power by gasifying the feedstock within gasifiers into hydrogen and carbon monoxide containing streams. These gasifiers typically require oxygen at high pressures in which the oxygen is supplied as a supercritical fluid. Although there are many different types of gasifiers generally speaking, a low-grade carbon containing material in the presence of oxygen is converted to a hydrogen and carbon monoxide containing stream that can be further processed to be used as a fuel in the generation of electricity and/or as a source of hydrogen, or further processed to manufacture valuable products such as chemicals, fertilizers or liquid fuels. Additionally, steam is generated in such processing that can be further used to drive generators.
While such oxygen can be supplied by vaporizing liquid oxygen and then compressing the oxygen to pressure, the liquid oxygen can be pumped to a high pressure and then heated to a critical temperature at which the resulting oxygen product will exist as a supercritical fluid. Typically, the pumping operation is incorporated into a cryogenic air separation plant, although, it is possible that the pumping operation could be conducted independently of such a plant. In a cryogenic air separation plant that is used in producing the oxygen at pressure, air is compressed, purified and then cooled to a temperature suitable for its rectification in a distillation column system.
Although different distillation column systems exist for the rectification of air, a common system involves two columns, a high pressure column and a low pressure column that are thermally linked by means of a condenser reboiler. The air, after having been cooled to at or near its dew point, is then introduced into the high pressure column in which nitrogen is separated from the air to produce a nitrogen-rich column overhead and a crude liquid oxygen column bottoms. The crude liquid oxygen column bottoms is further refined in the low pressure column into an oxygen-rich liquid column bottoms and a nitrogen-rich column overhead. All or part of the nitrogen-rich column overhead produced in the high pressure column is condensed against boiling the oxygen-rich liquid column bottoms of the low pressure column to provide a reflux for both high pressure column and the low pressure column.
The liquid oxygen that is drawn from residual oxygen-rich liquid in the low pressure column is pumped to pressure and then heated in a multi-stream main heat exchanger that is used in cooling the air against one or more product streams, or in a separate heat exchanger dedicated to the heating of the oxygen. In either case, part of the air to be rectified is further compressed in a booster compressor and then used to heat the oxygen and then produce the high pressure oxygen product that can be used in a gasifier or other process requiring high pressure oxygen.
As can be appreciated from this discussion, the raw material used in producing the oxygen is the electrical power drawn, or steam consumed or fuel burned to produce the energy for compressing the air in the first instance and further compressing the air to vaporize the pumped oxygen. In this regard, since the cryogenic rectification is conducted at cryogenic temperatures and there exists thermal loss due to heat leakage, liquid products that are removed from the plant for storage, backup or merchant liquid sale and warm end losses, refrigeration must be imparted. This is commonly accomplished by further compressing part of the air to be separated and then expanding the air in a turboexpander with removal of the work of expansion. The resulting exhaust is then introduced into the distillation column system. There are other known processes for generating refrigeration in an air separation plant. The production of refrigeration represents a further energy requirement of the plant.
In order to produce oxygen at supercritical pressures, that is above pressures at which the oxygen will exist as a supercritical fluid when also, at a temperature that will set the physical state of the oxygen as a supercritical fluid, the energy expended in compressing the air must be at a minimum or near a minimum to make the production of the oxygen economically attractive. In 89 AIChE Symposium Series 294, “Modern Liquid Pump Oxygen Plants: Equipment and Performance”, No. 294 by W. F. Castle, BOC Process Plants, p 14, it is mentioned that as a rule of thumb, the pressure of the air has to be about 2.3 times that of the oxygen pressure that is required. A simulation was conducted over a range of oxygen pressures in which the oxygen was vaporized by compressed air in a heat exchanger operated at a 5° C. warm end temperature difference and at an approach or pinch of the heating and cooling curves of about 1.5° C. The results were presented in graphical form. In the curve shown in the graph, at an oxygen pressure at above 40 bar, the curve flattened out from a relationship in which the required air pressure was roughly twice the oxygen pressure. It was mentioned, however, in the paper that such curve did not represent optimum conditions for the best power consumption of the plant and such optimum conditions were not presented in the paper. It was also mentioned, that the heat exchanger for air pressures below 100 bar could be a conventional brazed aluminum plate-fin heat exchanger. However, at higher pressures, more expensive coiled heat exchangers would have to be used.
U.S. Pat. No. 6,430,962-B2 also considers the production of oxygen as a supercritical fluid. In this patent, the oxygen produced in a low pressure column of an air separation plant is pumped to a supercritical pressure and then vaporized in a brazed aluminum plate-fin heat exchanger. It is mentioned that the more narrow the temperature difference between the oxygen and the air at the warm end of the heat exchanger, the lower the thermal stress within the heat exchanger. Two cases were compared, one at 0.61 Mpa less than the critical pressure of oxygen, 5.043 MPa and another far above the critical pressure, a pressure of 8.14 MPa. From the comparison, it was determined that at the subcritical pressure, the warm end temperature difference was large, 40° C. and at the high pressure, the warm end temperature difference was 12° C. This lower temperature difference would reduce the thermal stress in the heat exchanger and allow the use of a brazed aluminum plate-fin heat exchanger in such applications. However, nothing is said in this patent regarding the most efficient operation of the plant with respect to the electrical power used in compressing the air. Further, there are no details given regarding the design of the heat exchanger itself.
In U.S. Pat. No. 7,219,719 B2, a brazed aluminum plate-fin heat exchanger design is disclosed that is designed to be used at oxygen pressures above 100 bar. In this patent, straight extruded fins are used in the high pressure channels, having a sufficient thickness to withstand such high pressures. It is mentioned that the ratio of the mean fin thickness to the geometric pitch, or spacing between adjacent fins is preferably greater than 0.2 and less than 0.8. However, as will be discussed, such a design would lead to an inefficient heat exchanger with respect to the size required to accomplish the necessary heat exchange between air and pumped oxygen streams. U.S. Pat. No. 6,951,245 discloses another brazed aluminum plate-fin heat exchanger that employs straight fins.
As will be discussed, among other advantages, the present invention provides a method of producing an oxygen product as a supercritical fluid that involves heating a pumped liquid oxygen stream with the use of supercritical pressure air in which a relationship has been determined that will allow the power consumed by the air compressor to be minimized and that can be used in connection with a heat exchanger design that will incorporate a more efficient fin design than disclosed in the prior art.